Thermal mass transfer printing employs a donor sheet-receptor sheet system, whereby a thermal print head applies heat to the backside of a donor sheet in selective imagewise fashion. The images are transferred to the receptor sheet either by chemical reaction with, or mass transfer from, the donor sheet. Mass transfer systems provide for the transfer of colored material directly from the donor to the receptor sheet, with no color-forming chemical reaction occurring.
In wax thermal (mass) transfer printing, an ink or other record-forming material in admixture with a wax compound is transferred from a donor material such as a carder ribbon to a receptor sheet by applying heat to localized areas of the carrier. The wax/ink mixture on the carrier ribbon melts or softens, preferentially adhering to the receptor sheet, which may be either paper or transparent film. In the case of paper, the receptor sheet has more surface roughness than does the carrier, so ink transfer is largely achieved by a physical interlocking of the softened wax and ink with the paper fibers.
The transfer of a marking material to a receptor sheet film such as transparent polyester, differs in that the surface of the film is very smooth. Here, wetting of the film surface by the softened wax/ink mixture must be adequate in order to provide preferential adhesion of the wax/ink mixture to the receptor rather than to the donor sheet. The transfer of single pixel dots is particularly sensitive to differences in adhesion because some of the heat input at the individual dot is dissipated into the surrounding ink mass, decreasing the temperature of the dot and lessening its ability to transfer.
A number of polymeric coatings placed on the receptor sheet have been claimed to improve ink transfer, including polyester, polycarbonate, polyamide, urea, and polyacrylonitrile resins, saturated polyester resins, stearamide, and poly(alkylvinylethers), poly(meth)acrylic esters, polymethylvinylketone, polyvinylacetate, and polyvinylbutyral. In general, these polymeric coatings have a somewhat higher degree of adhesiveness than the transparent film substrate. This accounts for an increased receptivity of the coating as compared to the substrate. Heat transfer from the printing head to the coating increases adhesiveness even further.
Examples of this type of coating are disclosed in U.S. Pat. No. 4,678,687, issued to Xerox Corporation, which relates to thermal transfer printing sheets useful as transparencies wherein a polymeric coating is applied to a receptor substrate. The coating can be a poly(vinylether), poly(acrylic acid ester), poly(methacrylic acid ester), poly(vinylmethylketone), poly(vinylacetate) or poly(vinylbutyral). The coating allegedly provides increased resolution as compared to an uncoated substrate by increasing the adhesion of the transferred ink or dye to the receptor printing sheet. The coating composition is approximately 100% of the recited polymers.
A problem axises with these compositions when the tackiness of the coating is high enough to cause feeding problems and jamming of the printer due to adhesion either between receptor sheets, or between the receptor sheets and the printer rollers. High tackiness can also result in excessive wax transfer from the donor which, in the case of transfer of single pixels, results in unacceptable half tone images due to bridging of individual half tone dots. Excess tackiness also results in fingerprinting and blocking.
Problems also can arise due to electrical charge build-up on the sheets. This build-up can occur during convening, jogging of film stacks and during film transport in the printer during the printing process. Such build up can cause misfeeds, printer jams, and multiple sheet feeding due to static cling.
U.S. Pat. No. 5,169,468, issued to Graphics Technology International, Inc., also provides a receptor sheet for wax thermal mass transfer printing using polymeric materials for the image receptor layer. The '468 patent teaches the use of a poly(alkylvinylether) and another polymer having a higher glass transition temperature which results in good image quality. The receptor sheet described does not perform as well as one might like when a high speed printer is used.
U.S. Pat. No. 4,686,549, issued to 3M Company, relates to a receptor (i.e., acceptor) sheet having a wax-compatible image receptive layer. This layer has a critical surface tension higher than that of the donor sheet, to aid in wetting the image receptive layer, and a Vicat softening temperature (as measured by ASTM D1525 (1982))of the polymers forming the image receptive layer of at least 30.degree. C. up to 90.degree. C. to prevent tackiness of the receptor sheet at room temperature. At softening temperatures below 30.degree. C., according to this patent, problems arise such as fingerprinting and blocking of stacked film. The image receptive layer according to the '549 patent may contain a blend of wax and various polymers.
Polymeric coatings with a 30.degree. C. to 90.degree. C. softening point generally do have the advantage of minimal handling problems, as suggested by the above patent. The disadvantage is that such coatings are suitable for use only with selected combinations of printers and donor sheets. If, for example, the melting point of the wax on the donor sheet is above a specified maximum for a given printer, an insufficient amount of wax may be transferred to the receptor sheet. Likewise, if the particular printer does not provide sufficient heat energy, the heat transfer from the donor sheet to the receptor sheet, via the wax, may not increase the tackiness of the image receptive layer sufficiently for adhering the wax to the receptor sheet, even if the wax does melt sufficiently for transfer. The result is, inter alia, poor fine line reproduction. It has also been found that when high speed printers are used with the image receptive layers of U.S. Pat. No. 4,686,549, these receptive layers with a softening point of 30.degree. C. to 90.degree. C. do not provide adequate print quality.
A receptor sheet, particularly one applicable for wax thermal transfer printing, which can avoid the foregoing problems often encountered with the use of polymerics and other materials previously tried for acceptor/receptor sheets would be of great value to the industry.
Accordingly, it is an object of the present invention to provide a receptor sheet for wax thermal transfer printing having improved wax receptivity.
It is still another object of the present invention to provide a receptor sheet for wax thermal transfer printing which is particularly adapted to faithful reproduction of pixel dot image formation.
It is another object of the present invention to provide a receptor sheet for wax thermal transfer printing which provides wider printing latitude.
It is still another object of the present invention to provide a receptor sheet for thermal imaging which has a reduced tendency to jam the printing mechanism.
It is another object of the present invention to provide a novel receptor sheet for mass transfer imaging.
It is yet another object of the invention to provide an receptor sheet, as above, which maintains the above characteristics yet which can be used with high speed printers.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.